Before cellulose ester dope can be spun into fine filaments, the undissolved and partially dissolved particles and other contaminants must be removed. Typically this is accomplished with plate and frame filter presses which are slow, expensive and labor intensive.
The improved process provided by this invention is an adaptation of the commonly known cross flow filtration process which has wide application in the beer, wine, and pharmaceutical industries and is easily applied to batch type processes. Normally the fluid to be filtered has a low viscosity and low solids loading.
Cellulose ester dope is a thick, viscous liquid which is quite shear sensitive, that is, when sheared the viscosity is lowered. For example, when the dope is pumped through a tube, the layer of dope along the tube wall is subjected to high shear stresses, hence the viscosity of that layer is reduced. This results in a plug flow. Also experimental results indicate that particles in a high viscosity fluid flowing in a tube tend to migrate away from the high shear region and towards the center of the tube. If the tube wall is porous, then the lower viscosity fluid produced in the wall boundary area will exude through the wall while most particles will be retained with the parent flow, thereby concentrating the parent flow. If the parent flow is continually circulated in the filter tube, the parent flow would become more and more concentrated as filtrate is removed from the parent flow. If raw dope is introduced as a sheath around this parent flow just prior to the filter, the filter could separate the gels and contaminates from the raw dope. Of course not all particles are contained in the parent stream and some will adhere to the filter wall. These particles will eventually start blinding the filter. When this occurs, most of these particles can be removed by a simple backflushing of the filter wall with the clean filtrate. After several backflushings, sufficient particles may be tightly bound to the filter wall and a more thorough cleaning may be required.
Of interest are U.S. Pat. Nos. 4,218,314 and 4,313,830, as well as publications mentioned hereinafter.